The GPS has evolved to the point where its accuracy and capabilities have been shown potentially to be adequate for aircraft landing operations, land surveying and other present and potential applications beyond basic navigational uses. However, in a number of such applications multipath error in reception of the GPS signals is the principal limitation in achieving the full accuracy potentially available in use of the GPS signals.
In GPS application for aircraft precision approach and landing guidance, for example, a key element is the use of Differential GPS (DGPS). As proposed for DGPS, a reference receiver station is located near an airport runway and, ideally, may service all runways at one airport and potentially several airports in a local sector. The function of the DGPS reference receiver is to provide corrections for ionospheric, tropospheric and satellite clock and ephemeris errors. The ground station would utilize an antenna with an accurately determined phase center to measure the local error in reception of the satellite transmissions. This error information, transmitted to an aircraft preparing to land, would permit on-board error correction. With full error correction, the accuracy inherent in the GPS signals can be more fully utilized.
Multipath error has been determined to be the principal limitation in achieving the degree of vertical accuracy required for aircraft approaches and landings under conditions of limited visibility. Multipath errors resulting from ground reflections are fundamental, however lateral multipath effects (as caused by buildings, for example) can also cause substantial errors. The ground multipath effects at the aircraft and at the ground reference point are both important considerations. With respect to the aircraft, there is little opportunity for improvement of the aircraft antenna characteristics to suppress multipath, because of the wide coverage required to enable signals to be received from at least four satellites and to accommodate aircraft roll and pitch. Aircraft motion does provide some benefit in averaging ground reflection errors, however the potential for significant error remains. Multipath errors in GPS application for aircraft approach and landing are considered in greater detail in the inventor's article entitled "GPS Autoland Considerations", in IEEE AES Systems Magazine, pages 37-39, April 1993.
A variety of forms of antennas have been considered for GPS applications. In addition, techniques such as use of corrugated ground planes, or location of the antenna on a circular ground plane positioned in close proximity to the ground, have been suggested in order to reduce ground reflections. However, these techniques do not fully solve the ground and lateral multipath problems. In addition, such non-elevated antennas have inherent disadvantages, such as the limited coverage area and the need for protection against flooding, dirt, debris and snow build-up, and protection against damage from airport traffic and ground maintenance activities.
It is therefore an object of this invention to provide antenna systems having a circular polarization characteristic (e.g., right circular polarization) at all directions horizontally and upward from a plane (e.g., from the horizon to the zenith) and having a sharp cut-off characteristic beginning at the horizon or at a limited angle below the horizon.
It is a further object to provide compact and economical GPS systems utilizing stacked arrays of dipoles.
It is an additional object to provide antenna systems having a circular polarization characteristic in all directions above a cut-off angle, which characteristic is effective to discriminate against reception of ground and lateral multipath signals which have undergone polarization reversal upon reflection. Further objects are to provide new and improved antenna systems usable for a variety of GPS and other applications.